ERC-8340: Transaction Metadata Encoding

A new draft ERC titled 'Transaction Metadata Encoding' (ERC-8340) was proposed via a GitHub PR by chunter-cb, defining the byte layout for the opaque metadata field introduced by EIP-8130's account-abstraction transaction type. The metadata is a single deterministic CBOR value (a text string, byte string, map of reserved keys such as attribution, memo, commitment, and scope, or an array of these), with map keys interoperable with ERC-8021 schema 2, and any value may be recursively replaced by a salted commitment for selective disclosure. The ERC depends on ERC-5792 and EIP-8130, specifies an offchain proof package with verification and delivery protocol, and includes an ERC-5792 metadata capability that supersedes dataSuffix.